A turning speed idiosyncrasy observation

[Ted Evans]

A question from a rookie about turning speed. A smallish bowl blank, about 8 x 3 spalted maple, a little off balance, starts to vibrate a 1642 at approximately 700-750 rpm. It is not trying to walk but is noticeable, run speed up to 1050-1100 and it becomes as smooth as can be. I have observed this scenario on perhaps three different pieces now, not all the same speeds but the same reaction. Is this an anomaly and if not, any explanations?

Thanks.

[John Keeton]

Ted, I don't have a 1642, but I have found that often a blank will vibrate out of balance worse at a somewhat lower speed. However, I would note that the 1100 rpm is real close to the max speed for that size of turning.

8 x 1100 = 8,800
Safe range = 6,000 to 9,000

[Tony De Masi]

Ted, it's not a problem with the 1642. I've noticed the same thing with my 3520. I don't have an explanation for it but it does happen.

Tony

[Larry Marley]

I believe the oscillations vary with harmonics. I have experienced the same thing.

[Steve Schlumpf]

Ted - I have a 1642 and basically can not run anything in the 700 rpm range because of vibrations. I either turn at 650 rpm or move it up to 850 or slightly higher depending on the size of the turning.

Personally, I believe it has to do with the frame of the lathe, cast iron and the given rpm range generating harmonics that cause everything to start to vibrate. I wouldn't worry about it - but I also wouldn't turn an out of balance blank much above 850 rpm - just to be on the safe side.

[Jerry Marcantel]

Ever notice while cruising down the hyway that your vehicle has an annoying vibration at the posted speed limit, but about 5 miles over, it settles down to running smooth again. It also runs smooth 5 miles under the limit, but why drive slower????? The laws of physics, I suppose, plus the State's laws that you're in and speeding..... Jerry (in Tucson)

[Jeff Willard]

Think of it like this. The machine is "tuned". At a specific speed (frequency), it will vibrate (oscillate). No matter what. It will also vibrate at multiples (harmonics) of that frequency. Deviate from the frequency, up or down, a little bit, and the vibration stops. Every lathe that I have ever turned on has exhibited this characteristic.

Congratulations. You now also possess the depth of my scientific knowledge of sound and vibration transmission:eek:. Be careful. A little knowledge can be a dangerous thing.

[Ted Evans]

Thanks to all for the info. It is not something that I was concerned about, just curious as I had not seen this mentioned before.

Steve - I am a coward at heart so if starts moving very much, I back off in a hurry.

Good point Larry, I kinda of thought it could be something like that.

[Allen Neighbors]

Yep. It's harmonics. A little faster and it quits vibrating, and then a little faster and it begins again... Stay safe!

[Curt Fuller]

I don't know any technical reason but I'm sure it's similar to having a tire out of balance on your car. At certain speeds it drives you nuts, at others you can feel it.

[Ken Fitzgerald]

What you are experiencing is the results of the natural mechanical resonant frequency of the bowl you are turning.

Above or below that frequency/RPM, the vibration should be less and therefore not experience the problem or at least not as bad.

This is the same reason tires and wheels are dynamically balanced so that you don't experience the problem while driving your car down the highway.

We do very similar thing on the revolving portion of a CT scanner. It has to be balanced or it would try to walk the gantry across the floor while performing a CT scan on a patient. With CT scanners we actually monitor the current necessary to spin the rotate frame. From the current variations, you can determine where the variations are in weight on the rotate frame and apply or remove weights accordingly.

[Dave MacArthur]

Jeff Willard has it right. It's not soley the bowl blank OR the machine, but the combined system will have a resonant frequency. Just like a tuning fork will vibrate at a certain pitch no matter how hard you hit it or what angle, in any system (a bridge, an aircraft airframe, a drum) as force waves travel through the various parts they will reflect internally and bounce back and forth. If the return wave is out of sync with the incoming wave (high point matches the low point) the vibrations will cancel each other out. However if the high points coincide they will REINFORCE each other and make the system vibrate even more. Every system has one or several "resonant frequencies" at which the internal vibrations are reinforcing each other making the system oscillate more and more. In engineering complex systems this must be taken into account. In the F16, the various parts are all tested individually and as collected systems (usually via computer modeling) to make sure that in certain speed and G-loading the wings won't start oscillating and rip themselves off, or a bulkhead won't start flexing.

This is why soldiers do not march in unison over bridges but "rout step" to keep from inputting low-frequency pumping oscillations--there are many cases where bridges have collapsed due to this. A wind-input self-reinforcing oscillation can be seen in a video you can google when the Tacoma Narrows bridge began oscillating due to wind through the wires of the bridge, and ripped itself apart.

This is the same phenomenon that enables lasers to function, as waves in a particular frequency are passed through a medium of a certain integer multiple of that frequency, building up higher and higher amplifications as wave-tops reinforce each other, eventually emitting out the end of the reflector chamber.

This is the same phenomenon that allows tuning of a radio frequency and transmission of various frequencies at multiples of half-wavelength antenna lengths.

Same idea that lets you filter frequencies out with a tuner on an electric guitar, hit harmonics on a lightly tapped guitar string, and get pleasing notes and chords on musical instruments by dividing the string into wave-length integer chunks or hitting self-reinforcing combination of notes where each note is a multiple of the same frequency.

Same thing that lets noise-canceling headphones work, and allows small motor/vibrators mounted on space-station structures cancel out these same resonant vibrations to keep the space-structure from ripping itself apart in an environment where no gravity/air friction or other damping forces are present to inhibit growth of resonant oscillations.

For that matter, it's the same idea that allows flourescent lights to work and quantum mechanics-- it's all the same idea that despite the input force nature, systems will output or vibrate at a set self-reinforcing frequency.

Think about that next time you're turning a bowl/contemplating the universe! ;)

[Jeff Willard]

Thanks Dave. I think:D.

Yestiddy I couldn't spell mechanical engineer. Today I are one.

BTW, Raffan mentioned it in one of his books. I just sorta regurgitated it:eek:.

[Ted Evans]

Gee Dave, I knew all of that, I only wanted to see if others did. Err, I admit that I had forgotten some of it. What did you say anyway.

Thanks

[Dave MacArthur]

Heh... Sorry :o ... I got overly excited there! I'll admit, I have a degree in mathematics, and one in electrical engineering; and an interest in something called fourier series, basically breaking down total-system oscillations into the component sub-oscillations (frequencies). In EE you can use this to "filter" power and sound and signals to remove noise and unwanted signal. I mean, it's the KEY to vibration-free equipment, doesn't EVERYONE love that stuff? heh

Well, for a minute there I thought everyone loved that stuff, and I'd found a new friend:rolleyes:

Uhm...Now you've got me worried... what are YOU thinking about while you're turning? ;)

[Brian Ashton]

you know it's time to back off the speeds REAL QUICK (at the very least stay out of the line of trajectory) when you can get a 1000lb lathe bouncing around the shop from an out of balance piece of wood.

[John Keeton]

Uhm...Now you've got me worried... what are YOU thinking about while you're turning? ;)Dave, I just hope as my head spins over reading all this stuff, the "Oh crap" moments are cancelled out by the "Ahh, that worked" moments, such that it doesn't reach a resonant frequency and blow up on me!!!:D:D